RS.CLIPPY.BAD_BIT_MASK
Expressions of the form `_ & mask == select` that will only ever return `true` or `false`
This checker is a Clippy lint created by The Rust Project Contributors. The documentation shown here is a copy of the original documentation for: bad_bit_mask. Copyright ©2025 The Rust Team. All rights reserved.
What it does
Checks for incompatible bit masks in comparisons.
The formula for detecting if an expression of the type _ <bit_op> m <cmp_op> c (where <bit_op> is one of {&, |} and <cmp_op> is one of
{!=, >=, >, !=, >=, >}) can be determined from the following
table:
| Comparison | Bit Op | Example | is always | Formula |
|---|---|---|---|---|
== or != |
&
|
x & 2 == 3
|
false
|
c & m != c
|
< or >= |
&
|
x & 2 < 3
|
true
|
m < c
|
> or <= |
&
|
x & 1 > 1
|
false
|
m <= c
|
== or != |
|
|
x | 1 == 0
|
false
|
c | m != c
|
< or >= |
|
|
x | 1 < 1
|
false
|
m >= c
|
<= or > |
|
|
x | 1 > 0
|
true
|
m > c
|
Why is this bad?
If the bits that the comparison cares about are always
set to zero or one by the bit mask, the comparison is constant true or
false (depending on mask, compared value, and operators).
So the code is actively misleading, and the only reason someone would write this intentionally is to win an underhanded Rust contest or create a test-case for this lint.
Example
if (x & 1 == 2) { }